We’ve seen that squid can, in fact, fly through the air using jet propulsion. It

Question

We’ve seen that squid can, in fact, fly through the air using jet propulsion. It’s been proposed that some species of squid migrate by doing a series of such flights. As much as I love the idea, there’s one big problem: Jet propulsion is very inefficient. Let’s use typical data to see why. A stationary 1.5 kg squid (not including the ejected water) explosively ejects 0.10 kg of water at a speed of 45 m/s. For this case,

a. How fast is the squid moving after it ejects the water?

b. What is the final kinetic energy of the squid?

c. What is the final kinetic energy of the water?

d. For this process, define “what you get” and “what you had to pay”, and compute an efficiency, without worrying about metabolic efficiency.

e. Now, add in a typical metabolic efficiency. What is the overall efficiency for this process?

Explanation / Answer

(A) aPPLYING momentum conservation,

0 = (1.5v) - (0.10 x 45)

v = 3 m/s

(B) K = m v^2 / 2 = 1.5 x 3^2 /2 = 6.75 J

(C) KE = 0.10 x 45^2 /2 = 101.25 J

(D) what we get = 6.75 J

what we had to pay = 6.75 + 101.25 = 108 J

efficiency = 6.75 / 108 = 0.0625

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